RFR: 8313809: String template fails with java.lang.StringIndexOutOfBoundsException if last fragment is UTF16

[Jim Laskey]

The last fragment of a string template does not get it's coder added to the mix.

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Commit messages:
 - 8313809: String template fails with java.lang.StringIndexOutOfBoundsException

Changes: https://git.openjdk.org/jdk/pull/15195/files
 Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=15195&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8313809
  Stats: 45 lines in 2 files changed: 44 ins; 1 del; 0 mod
  Patch: https://git.openjdk.org/jdk/pull/15195.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/15195/head:pull/15195

PR: https://git.openjdk.org/jdk/pull/15195